Nowadays, most bicycles are equipped with a transmission system, and this transmission system makes it more convenient to ride the bicycle when a rider climbs a steep hill or drives the bicycle at a high speed. However, the conventional transmission system has a shift lever that is positioned on either one side of a bicycle steering bar's handle or one side of the bicycle frame. To change the bicycle speed, the rider has to select a gearshift during steering. Therefore, it is inconvenient to frequently change the driven gears to obtain the desired bicycle speed. If the rider puts the shift lever on an inappropriate position, an incorrect gear engagement takes place between the bicycle chain and the chain sprocket, which causes loud noise, abnormal wear and tear between gear sets, etc. In addition, the shift may distract a bicycle beginner from cycling, thereby causing a collision.